1. Field of the Invention
The present invention relates to a chain power-transmission system with mechanical tightener.
2. State of the Prior Art
In engineering applications calling for mechanical power transmission through a chain the problem of maintaining the correct chain tightening level over time arises. Indeed, the transmission chain tends to wear in the course of its useful life and to lengthen little by little. In addition, in the presence of mechanical systems which in addition to the changes in transmitted torque call for its reversal (as for example motorcycle primary transmission systems) the taut and slack chain branches are mutually reversed with each torque reversal (for example when changing from the motor pulling phase to the motor braking phase) and in addition chain length changes elastically in a reversible manner dependent on the torque transmitted.
Under these working conditions of the chain the tightener is required to perform various functions in order to make the chain work efficiently at all times and in particular:                it must provide for correctly tightening the normally slack branch of the chain in order to reduce its vibrations, flapping, noise and wear,        it must provide for recovering the irreversible lengthening of the chain due to its natural wear in time,        it must provide for recovering the reversible elastic lengthening of the chain with the changes in tension to which it is subjected in operation, and        it must be capable of opposing movements or shifting which the normally slack branch of the chain would do upon reversal of the torque transmitted while withstanding the high stresses exerted by the chain thereon.        
A transmission chain not correctly tightened and excessively slack produces high noise of the system and continuous flapping of the chain and these phenomena can cause considerable distress to users of the machine and considerable shortening of the useful life of the whole transmission system.
To solve the problem of chain tightening, one of the systems used is the tightener with articulated kinematic parallelogram mechanism. These systems typically comprise a tightening shoe connected to a support base by means of a connecting rod. The shoe is in sliding contact with a branch of the transmission chain and is thrust progressively against it to keep it in tension when the chain wears and becomes gradually slacker. The prior art quadrilateral tightening systems, while allowing recovery of a considerable chain lengthening, are not suited to use with transmission systems calling for reversal of the transmitted torque as for example motorcycle transmission systems. Indeed, in quadrilateral tighteners, the tightening shoe is thrust by a spring towards the chain to tighten it but is not capable of opposing chain movement when a transmitted torque reversal occurs.
To solve the problems deriving from reversal of the torque transmitted by the chain, systems comprising tightening shoes hinged at one end (traveling circular trajectories) or prismatically guided shoes (traveling rectilinear trajectories) having various nonreturn systems and thrust against the chain with spring or hydraulic action have been designed for example for motor distribution chains. But these systems allow compensation for small chain-lengthenings and opposing rather small torque reversals. They could not for example meet the requirements of a primary transmission for motorcycle use where it is necessary to be able to recover the considerable lengthenings of the chain due to wear, excessive elastic lengthenings thereof with changes in pull, and very high torque reversals when the motor changes from pulling to braking condition.
The general purpose of the present invention is to remedy the above-mentioned shortcomings by making available a power-transmission chain tightening system usable in mechanical systems calling for reversal of high transmitted torques and occupying little space and ensuring adequate and correct chain tightening level at any motor speed condition even when it is very worn and lengthened.